1. Field of the Invention
This invention relates to a tape takeup system for taking up tape in continuous length while controlling the tension applied to the tape.
2. Description of the Prior Art
As a tape takeup system which is used for slitting tape like magnetic tape in continuous length, or for rewinding such tape, there has been known one in which the speed of the tape feed roll is controlled and at the same time the torque of the takeup roll is controlled to control the tension applied to the tape.
In order to control the tension on the tape, the tension presently applied to the tape must be detected. As a tensiometer for detecting the tension on tape, there has been known one disclosed in Japanese Unexamined Patent Publication No. 3(1991)-80456. As shown in FIG. 9, the tensiometer comprises a tension detecting roller 2 around which a tape 1 is passed, a tension arm 3 one end of which is supported for rotation on the tension detecting roller 2 at the center thereof and the other end of which is fixed to an angle detecting device 4 at the center thereof, and a spring 5 which urges downward the tension arm 3.
When the tension applied to the tape 1 while the tape 1 is running increases, the tension detecting roller 2 is moved upward overcoming the force of the spring 5, and the tension arm 3 rotates the angle detecting device 4 in the direction of arrow A. The angle detecting device 4 detects the angle by which it is rotated and outputs a detecting signal to a tension control section. When the tension applied to the tape 1 reduces, the members move in the reverse direction and the angle detecting device 4 detects the angle by which it is rotated in the direction opposite to the arrow A and outputs a detecting signal to the tension control section.
In such a tensiometer, the relation between the actual tension applied to the tape 1 and the level of the detecting signal of the angle detecting device 4 is as shown by the curves in FIG. 10. That is, depending on the initial length of the spring 5 and/or the spring constant of the spring 5, the relation changes as shown by the solid line or the broken line in FIG. 10. Accordingly, when the spring 5 is changed or the resiliency of the spring 5 changes with age, the relation between the actual tension applied to the tape 1 and the level of the detecting signal of the angle detecting device 4 changes.
In order to overcome such a problem, conventionally, the zero position of the detecting signal is adjusted or the output gain of the tensiometer is adjusted.
Recently, numerical control has come into wide use in various fields for the purpose of improving accuracy in control and reducing the cost. Also in the tape takeup system, this applicant has proposed in U.S. Pat. No. 4,977,466 to control the system by use of numerical control.
When the numerical control is employed in controlling the tension on the tape, the level of the detecting signal of the angle detecting device 4 is once converted into a numerical value and the numerical value is fed to the tension control section. The numerical value is converted into a value of tension on the basis of a predetermined formula or a predetermined table stored in a memory.
In the numerical control, change in the initial length of the spring or the spring constant can be compensated for by rewriting the formula or the table instead of zero position adjustment or the gain adjustment.
However, when the operator measures the signal level for a reference tension and writes the formula or the table calibrated on the basis of the result of the measurement in the memory each time the spring is changed or every predetermined time, the labor involved is great and at the same time the calibrating time cannot be shortened.